Embodiments of the disclosed technology relate to a manufacturing method for a thin film transistor with a polysilicon active layer.
Polysilicon has relatively high carrier mobility (10-300 cm2IVs) due to its regularly arranged atoms. Meanwhile, thin film transistors (TFTs) with a polysilicon active layer have a relatively high driving current, which can reduce the response time of liquid crystal in a liquid crystal display employing such TFTs. Thus, the volume of the TFTs can be reduced and the aperture ratio of the liquid crystal display can be increased so as to obtain higher brightness and higher resolution. Such advantages of the TFTs with polysilicon are also favorable for an active matrix organic light-emitting display (AMOLED). Instead of a voltage driving manner of liquid crystal displays, a current driving manner is employed in an AMOLED; in such a case, only the polysilicon TFT can meet its requirements. In addition, a remarkable advantage of polysilicon is that a driving integrated circuit (IC) using polysilicon can be integrated onto a panel, even in a system on glass (SOG), so that the product has advantages such as light weight, slim shape, and low power consumption.
A conventional manufacturing method for polysilicon is as follows. A layer of amorphous silicon (a-Si) is firstly deposited onto a glass substrate, and then the amorphous silicon is crystallized through an annealing thermal treatment. However, this method requires long time of annealing at a temperature higher than 600° C., and is not suitable for a glass substrate of a display. Furthermore, it has been found that some metals can function as catalysts for promoting crystal growth, and the crystallization temperature of the a-Si can be decreased by depositing a layer of metal on the a-Si. This method is called metal-induced crystallization (MIC) method. The MIC method can decrease the crystallization temperature to 500° C. or less. However, there is a serious problem when the MIC is applied to manufacture TFTs, i.e., the metal atoms tends to remain in the channel regions of TFTs as impurity atoms, which leads to current leakage in the channel regions and deteriorates the characteristics of the TFTs.
In addition, another metal-induced crystallization phenomenon is found, in which the crystal can laterally grow by 100 μm or more towards a region without being covered by the metal layer. This phenomenon is called as metal-induced lateral crystallization (MILC). The MILC can be performed under the induction effect by various metals such as nickel, and the crystallization temperature can be less than 500° C. In this case, strip shaped crystal grains can be formed with a relatively large size.
FIGS. 1A and 1B are schematic views showing a manufacturing method for a polysilicon TFT by a MILC process in the prior art. Referring to FIGS. 1A and 1B, the manufacturing method comprises the following steps. First, a buffer layer 102 is deposited on a glass substrate 101 through a plasma enhanced chemical vapor deposition (PECVD) method. The material of the buffer layer may be, for example, silicon dioxide (SiO2). Then, an amorphous silicon layer 103 is deposited on the buffer layer 102 through a PECVD or a low pressure chemical vapor deposition (LPCVD) method. Subsequently, an inducing metal layer of nickel is deposited on selected positions on the amorphous layer 103 (for example, positions for forming source/drain regions of the TFT structures in the subsequent steps) through a sputtering method. Finally, an annealing treatment is performed, generally for the annealing time of 0.1-10 hours. During the annealing process, MIC occurs firstly in the regions where the amorphous silicon and the metal nickel contact with each other directly so as to form MIC polysilicon regions, which are indicated by “103S” and “103D” in FIG. 1B. Subsequently, the polysilicon crystal grains laterally grow into amorphous regions not contacting with the metal nickel directly so as to form a MILC polysilicon region 103C as shown in FIG. 1B. Because of not contacting with the metal directly, the MILC polysilicon region has a much lower metal impurity concentration than the MIC polysilicon region.